In projection electron beam lithography, precise control of the placement of the electron beam is required in order to ensure that the image is constructed without distortion and aligned to a prior process level. Precise control of the electron beam placement is difficult because electron beam placement depends on many factors.
One of these factors is a wafer distortion response to the heating action of a projection electron beam lithography beam, ranging up to many hundreds of nanometers, depending on conditions. Correction schemes include a model-based predictor for sub-field center placement adjustment. The algorithm implemented by the model-based predictor controls the writing of a matched dynamic distortion with an accuracy of about 1% or better for the largest, long-length-scale effects of approximately 500 nm.
Other factors in addition to a predictable heating response, such as beam drift and wafer-to-chuck contact variation, also affect placement accuracy. Their effect may be either random or very difficult to correctly model.
As stated above, wafer-to-chuck contact may have an effect on the response that requires enhancement to a basic predictive model. Modeling and experiments have both demonstrated the desirable result that good thermal contact to the chuck (˜150 W/m2K) can lower the accumulated size of the wafer-heating response by a factor of roughly 10, thus enlarging the fractional correction error tolerance similarly. However, there are several factors, such as wafer-flatness, particle tolerance, frictional contact, and pulling-force that may remain variable or random despite efforts in chuck design. Realistically, the chuck design process can only reduce frictional influences on the heating response to a form of chuck-coordinate-system drift that is slow and indistinguishable from beam drift. Since important parameters in the predictive model may be variable from wafer to wafer, prediction alone is not sufficient for full correction of beam placement.
Further, it is difficult to perform the complex model computation required to determine correct beam placement in a short period of time.
The only alternative to prediction is measurement. The obvious primary measurement of beam placement involves an alignment mark sensing process. The use of a re-alignment strategy, or some variation of local alignment, is a common approach to dealing with drift in many other electron beam lithography applications, such as mask-making and direct-writing. This often involves time-consuming actions like extra stage motions that detract from throughput, but this can be a tolerable situation when making relatively few high-value exposures.
In the area of production wafer-level lithography using SCALPEL, throughput is a concern even without the use of local alignment or complex re-alignment strategies. Hence, re-alignment is not a suitable correction strategy for a high-throughput SCALPEL tool.
Based on the above, it is clear that an enhancement to the predictive models used for beam placement correction is desirable, making use of alignment mark sensing and efficient computation.